1. Field of the Invention
The present invention relates to a solar power generation system having a cooling mechanism. More particularly, the present invention relates to a solar power generation system in which a solar cell is installed and which is provided with a cooling mechanism capable of cooling said solar cell depending on an output of said solar cell.
2. Related Background Art
In recent years, as an energy source which is safe and does not bring about a load to the environment, a solar power generation system in which a solar cell is used has been spotlighted. However, such a solar power generation system is necessary to be more advantageous in comparison with conventional power generation systems also in terms of the economical viewpoint in order that it can be more widely used. Because of this, various studies have been conducted in order to develop a solar cell having an improved photoelectric conversion efficiency and capable of being produced at a reasonable production cost and which makes it possible to establish a solar power generation system having a high power generation efficiency at a reasonable cost.
Incidentally, in order for a solar cell to have a large output energy, it is important that the solar cell is made to have a large photoelectric conversion efficiency. Besides, it is important to contrive such that the generated energy of the solar cell is increased. In order to increase the generated energy of the solar cell, there is considered, for instance, a measure that the solar cell is maintained at a temperature which is as low as possible. Specifically, in the case where the solar cell is installed outdoors, when the solar cell receives direct sunlight, the temperature thereof is risen, where there is a phenomenon in that the effective power generation efficiency of the solar cell is reduced due to the temperature rise in comparison with that when the solar cell is maintained in a rated state (where the solar cell is maintained at 25xc2x0 C.). In order to prevent occurrence of this phenomenon, it is necessitated that the solar cell is maintained at a temperature which is as low as possible. In the case where the solar cell is exposed to direct sunlight in summer time, the temperature of the solar cell generally reaches 80xc2x0 C. or more, where when the solar cell is a silicon series solar cell (such as a crystalline silicon series solar cell or an amorphous silicon series solar cell), the temperature coefficient of the photoelectric conversion efficiency thereof is about 0.4%/xc2x0 C. (which is meant that the absolute value of the photoelectric conversion efficiency is reduced by about 0.4% per a temperature rise of 1xc2x0 C.) and because of this, the power generation efficiency thereof is reduced by more than 20%. Therefore, even when a silicon series solar cell having a sufficiently high photoelectric conversion efficiency should be used, unless the silicon series solar cell is adequately cooled, it is difficult for the solar cell to achieve a satisfactory power generation efficiency. Further, in the case where the solar cell is maintained at a relatively high temperature, heat load to the components thereof is increased and accordingly, the durability of the solar cell is deteriorated. Also in view of preventing the durability of the solar cell from being deteriorated, particularly in the case where the solar cell is installed outdoors, it is necessary to cool the solar cell so that the solar cell can be maintained at a temperature which is as low as possible.
Now, in recent years, from the viewpoint of attaining a solar power generation system having a high power generation efficiency at a reasonable cost, a solar power generation system having an optical concentration type solar cell provided therein has started receiving the public attention. In the case where such an optical concentration type solar cell is used, there are advantages such that the number of solar cells, which are the most expensive of the components constituting the solar power generation system, can be diminished. This situation makes it possible to attain a solar power generation system having a high power generation efficiency at a reasonable cost.
In a solar power generation system, even when a relatively small number of optical concentration type solar cells are used, light with a large intensity is impinged into the solar cells to generate a large voltage, where the proportion of the output power energy to the incident light energy, that is, the photoelectric conversion efficiency is improved. Thus, there can be achieved a relatively large power output. Specifically, for instance, when a case wherein a prescribed number of optical concentration type solar cells are arranged on a prescribed area is compared with a case wherein a prescribed number (which is the same as the former number) of solar cells which are not of the optical concentration type are arranged on a prescribed area (which is the same as the former area), the power outputted in the former case is significantly greater than that in the latter case. Even in the former case, in order to achieve a sufficient power output by sufficiently increasing the photoelectric conversion efficiency, it is necessary that an optical focusing system with a high magnification is adopted and a sun-tracking mechanism is provided. However, in this case, the temperatures of the solar cells are more increased in comparison with those when the optical focusing of sunlight is not performed and therefore, it is necessitated to more efficiently cool the solar cells.
In view of the above situation, there has been made a proposal of performing forcible (intentional) cooling for the solar cells in a solar power generation system in order to lower the temperatures of the solar cells. For instance, Japanese Unexamined Patent Publication No. Hei.9(1997)-213980 (hereinafter referred to as JP ""980) discloses a forcible cooling means for intentionally cooling the solar cells in a solar power generation system.
The term xe2x80x9cforcible cooling meansxe2x80x9d is meant a means for intentionally cooling an object to be cooled, which is distinguished from a means for cooling said object by way of spontaneous heat radiation or the like.
Particularly, the forcible cooling means disclosed in JP ""980 is of the method of performing continuous cooling, where the same energy is used in the forcible cooling for the solar cells regardless of whether solar irradiation is relatively large or small. However, this method has shortcomings such that when the cooling function is designed to conform the time when the solar radiation becomes maximum, excessive energy is consumed when the solar radiation is low and therefore, there entails a disadvantage in that the energy is wasted; and reversely, when the cooling function is designed to conform the time when the solar radiation is weak, there will entail a problem in that the solar cells cannot be sufficiently cooled when the solar radiation is strong.
In order to solve these shortcomings, Japanese Unexamined Patent Publication No. Hei.5(1993)-83881, Japanese Unexamined Patent Publication No. Hei.7 (1995)-36556, and Japanese Unexamined Patent Publication No. Hei.10(1998)-101268 propose a method wherein the temperature of a solar cell to be cooled is detected by a temperature-detecting means and when the detected temperature exceeds a prescribed value, a forcible cooling means such as a fan or the like is actuated to cool the solar cell. However, such a method has shortcomings such that the temperature-detecting means is specially provided and this makes the system costly; when a failure is occurred at the temperature-detecting means, the failure sometimes will cause a failure at the solar cell; and because the cooling effect of the forcible cooling means is always constant, the foregoing problems relating to excess and deficiency of the cooling extent cannot be sufficiently solved.
As a measure to solve such shortcomings as above described, Japanese Unexamined Patent Publication No. Hei.7(1995)-240532 (hereinafter referred to as JP ""532) proposes a method wherein a cooling fan is electrically serialized with a circuit extending from solar cell which are electrically connected with each other and cooling by means of the cooling fan for the solar cells is performed in proportion to a current value generated by the solar cells. However, in general, the cooling effect is not proportional to the power, voltage, current and the like required for the cooling. Therefore, the method described in JP ""532 has shortcomings such that when the cooling system is designed such that the cooling effect to the solar cells becomes optimum when the solar cells are in a maximum power generation state, excess or deficiency of the cooling is occurred at an intermediate stage between the case where the solar cells are in a non-power generation state and the case where the solar cells are in a maximum power generation state, where when the solar cells are excessively cooled, the cooling energy is wasted, and when the solar cells are deficiently cooled, excessive temperature rise is occurred in the atmosphere surrounding the solar cells to impart a detrimental effect to the system.
The present is aimed at solving the foregoing problems found on the solar power generation system in the prior art.
Another object of the present invention is to provide a solar power generation system (or a solar cell power generation system) having a solar cell arranged therein and which is provided with a cooling mechanism having a forcible cooling means (that is, a means for performing intentional cooling) which is capable of realizing a necessary cooling effect with neither excess nor deficiency for the solar cell by way of making use of an attribute of the forcible cooling means and is also capable of minimizing the equipment cost and the operation cost.
A further object of the present invention is to provide a solar power generation system having at least a solar cell provided therein and which is provided with a cooling mechanism, characterized in that said cooling mechanism has a cooling means (a forcible cooling means) for intentionally cooling said solar cell and a memory and operation means for memorizing or operating an optimum cooling and driving state of said cooling means with respect to an output of said solar cell, wherein said cooling means is driven based on an output of said memory and operation means.
In the solar power generation system of the present invention, an output of the solar cell is detected, and with reference to the detected output of the solar cell and a previously presumed cooling attribute of the cooling means, necessary cooling drive by means of the cooling means for the solar cell is performed, where the cooling of the solar cell can be realized with the consumption of the cooling energy to a necessary minimum extent. Now, the term xe2x80x9ccooling attributexe2x80x9d is meant a relationship between a cooling effect by the cooling means and a drive magnitude of the cooling means, specifically for instance, a relationship between a cooling effect(a temperature reduction magnitude) by the cooling means and a pumping flow rate (a flow rate of a fluid coolant) in the cooling means as shown in FIG. 5 which will be detailed later).
In the case where a cooling means in which a fluid (liquid) coolant is used is used as the cooling means, the flow state of the fluid coolant is nonlinearly changed depending on the flow velocity and therefore, by adopting such cooling drive method as above described, there are provided significant advantages, typically as will be described below.
(i) Because the output of the solar cell is used for the cooling drive, it is not necessary to use an extra means for detecting the temperature of the solar cell and therefore, the production cost of a solar power generation system can be diminished.
(ii) By performing the drive of the cooling means (the forcible cooling means) with reference to the cooling attribute of the cooling means, the cooling with neither excess nor deficiency can be always performed for the solar cell, where the energy required for forcibly cooling the solar cell can be limited to a necessary minimum extent.
In the solar power generation system of the present invention, to cool the solar cell installed therein by means of the foregoing cooling mechanism is performed typically in a manner as will be described below.
(1) An output of the solar cell is detected,
(2) a magnitude of a rise in the temperature of the solar cell is operated (computed) based on the detected output,
(3) the temperature rise magnitude computed is added to a prescribed estimate temperature of the solar cell to presume a temperature of the solar cell at that time,
(4) a temperature difference between the presumed temperature and a temperature range in which the temperature of the solar cell is intended to control is operated (computed),
(5) a forcible cooling dive magnitude for lowering the temperature difference by way of cooling is operated (computed), and
(6) the cooling means is driven to meet the forcible cooling drive magnitude by means of a control means.
Now, the memory and operation means used in the solar power generation system of the present invention may be independently provided. In the case where a memory and operation means is provided in other component(s) of the solar power generation system, it is possible that said memory and operation means is made to serve also as the memory and operation means used in the cooling mechanism. In this case, the production cost of the solar power generation system can be diminished. As aforesaid other component, in the case where the solar power generation system is connected with a commercial power supply system such that a dump power occurred in the solar power generation system is flown to the commercial power supply system, there can be mentioned a power converter (a so-called inverter) which converts a d.c. power into an a.c. power, and a charge-and-discharge control means (a so-called power controller) which is provided in a direct current power storage means such as a storage battery or the like which is connected to the solar power generation system. Besides, the solar power generation system may be provided with an equipment for observing, memorizing and indicating the power generation state thereof. The equipment in this case is provided with a memory and operation means for controlling the equipment. It is possible that this memory and operation means is made to serve also as the memory and operation means in the fore going cooling mechanism.